Barrel Length--Impact on "Effective" Twist?

Status
Not open for further replies.
rockhound758

rockhound758

Member
Joined
Nov 23, 2008
Messages
176
Location
Columbus, OH
Hey folks! I searched and didn't see information related to this so thought I'd post...

I'm looking at a Ruger Gunsite Scout, and barrel length options are 16.5" and 18" (twist rate in both barrels is 1:10).

I'm trying to decide on which barrel for the GSR. I will likely suppress it (although not always use the suppressor of course), which makes me think shorter is better. But if I want to be shooting heavier bullets with it, I wonder if the shorter barrel will have enough length to stabilize the heavier bullets.

I understand that it's good to match up twist rate with the bullet you're shooting (weight, length, etc.), but my question relates to barrel length. More specifically, does a shorter barrel effect the "effective" twist rate? In other words, is a 1:10 in a 16.5" barrel not "effectively" as fast as a 1:10 in a 22" barrel? It probably doesn't make any difference between 16.5" and 18", but I was curious about it and thought I'd ask the folks here.

This is probably overthinking it (and I'm also pretty sure my own accuracy issues would overshadow any problems related to stabilization) but I'd be interested in your thoughts.

Have a good one.
 
Last edited:
Yup, you're definitely over-thinking this. The bullet is turning as fast as it's going to turn by the time it has traveled through any portion of the barrel.

You really don't want your bullet "skidding" through the rifling as that would shred the metal off the surface of the bullet! So by the time it has passed out of the leade it has started to turn.

Now there is such a thing as gain-twist rifling where the rate increases through the barrel's length, but I doubt that's what you're talking about. Not a terribly common thing anyway.
 
Barrel length has nothing to do with it except how it affects velocity. It's RPM that stabilizes and it's velocity and twist rate that give you RPM

Bullet RPM = MV X 720/Twist Rate (in inches)
 
It bears reminding that MV is usually a function of barrel length (within practical limits), so barrel length *does* impact bullet stabilization in a roundabout sort of way.
 
Okay, thanks. I knew I was overthinking it but it was an interesting question to me. I just wanted to make sure I wouldn't be sacrificing anything much with the shorter barrel except velocity (which in this gun isn't a huge drop anyway). I'll assume a 168 gr. will stabilize fine in a 1:10 16.5" barrel.
 
Suppose you get 25 fps more velocity out of the longer barrel. The velocity of a 175gr Federal Gold match load will be around the 2500-2550fps neighborhood from that barrel. You are looking at 1% then. Not a big deal.
 
Suppose you get 25 fps more velocity out of the longer barrel
Click to expand...
Specific to the OP, that is correct. As a general observation, I've seen 200fps deltas between 16.5" barrels (77MkII Frontier) and 22" barrels (77Mk II Hawkeye) and *that* amount of velocity increase will prove potentially significant.
 
It bears reminding that MV is usually a function of barrel length (within practical limits), so barrel length *does* impact bullet stabilization in a roundabout sort of way.
Click to expand...

Yes.

In a .308, it is not likely to matter, at least not when comparing barrel lengths 1.5" apart. It could matter if you were comparing a 14" to a 30".

In a .223 Rem it more often matters because so many people shoot 14.5" to 16" carbines, and others shoot 22", 24", or longer bolt action barrels. There is a major velocity gain with most .223 loads going from 16" or less to 22" or more, and the heavier bullets that many people use (75-77gr) will often stabilize in the longer barrels due to the higher MV, but not stabilize in the short carbine barrels where they are coming out 300-400fps slower.
 
In a .223 Rem it more often matters because so many people shoot 14.5" to 16" carbines, and others shoot 22", 24", or longer bolt action barrels. There is a major velocity gain with most .223 loads going from 16" or less to 22" or more, and the heavier bullets that many people use (75-77gr) will often stabilize in the longer barrels due to the higher MV, but not stabilize in the short carbine barrels where they are coming out 300-400fps slower.
Click to expand...

Exactly. My 20" 1:9 savage SP10 will not stabilize Sierra 80 gr. MK's, but my buddies Savage 24" 1:9 will. The only thing I can think of is he is getting more velocity because of the extra 4".

Laphroaig
 
Sam1911 said:
Yup, you're definitely over-thinking this. The bullet is turning as fast as it's going to turn by the time it has traveled through any portion of the barrel.

You really don't want your bullet "skidding" through the rifling as that would shred the metal off the surface of the bullet! So by the time it has passed out of the leade it has started to turn.

Now there is such a thing as gain-twist rifling where the rate increases through the barrel's length, but I doubt that's what you're talking about. Not a terribly common thing anyway.
Click to expand...

Hmmm...

Twist rate does directly affect RPM, but so does velocity of the bullet as it travels down the rifled barrel.

Now, velocity of the bullet is a factor of many things, including bullet mass, type of powder used in the cartridge, and barrel length, right? So many bullets generally benefit from an increase in muzzle velocity from moderately longer barrels, right?

So, wouldn't that moderate increase in muzzle velocity also translate into a moderate increase in RPM?

If the instantaneous velocity of a bullet out of a 16.5 inch rifle barrel with a 1:10 inch twist rate is, say, 1700 fps, then the RPM would be 122,400 RPM.

If the instantaneous velocity of as bullet out of a 22 inch barrel with the same twist rate were 1900 fps, then the RPM would be 136,800 RPM. That's nearly a 12% increase in RPM.


Wouldn't the question then be whether or not this moderate change in RPM due to the moderate change in muzzle velocity cause any significant effects on bullet stability?

I don't know enough about bullet RPM vs stability to answer this. I imagine that for any given bullet mass, diameter, and geometry there is an optimal RPM/stability range, which directly translates to an optimal velocity range for any given twist rate.


Maybe you experienced reloaders can educate me on this.
 
So, wouldn't that moderate increase in muzzle velocity also translate into a moderate increase in RPM?
Click to expand...
Yes! That's what rbernie was getting at in post 4, and Laphroaig clarified in 11.

If you're on the borderline of what length bullet your barrel twist will stabilize, a significantly shorter barrel could cause the velocity to be lowered enough to not produce a fast enough spin.

So, while the bullet doesn't spin at a higher or lower rate per distance traveled, it CAN spin faster/slower over a given time interval, due to horizontal velocity differences.

So, he's probably overthinking it in his case, but as rbernie said, in a roundabout way it can be a real phenomenon.
 
Part of the reason I was asking the question originally was because if I do end up making some subsonic loads for a suppressor, I was thinking heavier bullets and lower velocity...I was wondering if under those conditions the shorter barrel would potentially not stabilize the bullet. Not really an issue if I'm shooting at shorter ranges anyway, I suppose, but it seemed an interesting question. Thanks for your replies.
 
a 1-10 twist should stablize 220's if I am not mistaken. I have seen several threads about sub-sonic ammo loaded for the Savage Hog Hunter, it is a 20" 1-10 twist
 
Barrel length has no affect on bullet stability. Barrel length may affect velocity which has an affect on RPM but it has no affect on bullet stability. If you were to launch a bullet from a 2 inch barrel with a 1:8 twist at 3000 fps, it will have the same RPM as if it were launched from a 24 inch barrel with a 1:8 at 3000 fps.

Saying barrel length affects bullet stability is the same thing as saying case capacity affects bullet stability. It would be like claiming a bullet, in a round about way, is more stable when fired from a 300 Winchester Magnum case than form a 308 Winchester case. It's very simple- Bullet RPM is determined by velocity and twist.

But it's not just RPM that makes a bullet stable in flight. How much precession does the bullet exhibit when it exits the muzzle? The highest velocity doesn't always give the best accuracy or best bullet stability
 
MistWolf said:
Barrel length has no affect on bullet stability. Barrel length may affect velocity which has an affect on RPM but it has no affect on bullet stability. If you were to launch a bullet from a 2 inch barrel with a 1:8 twist at 3000 fps, it will have the same RPM as if it were launched from a 24 inch barrel with a 1:8 at 3000 fps.

Saying barrel length affects bullet stability is the same thing as saying case capacity affects bullet stability. It would be like claiming a bullet, in a round about way, is more stable when fired from a 300 Winchester Magnum case than form a 308 Winchester case. It's very simple- Bullet RPM is determined by velocity and twist.

But it's not just RPM that makes a bullet stable in flight. How much precession does the bullet exhibit when it exits the muzzle? The highest velocity doesn't always give the best accuracy or best bullet stability
Click to expand...

Good points.

However, I'm now currently hung up on finding a pistol with a 2 inch barrel that can launch a bullet at 3000 fps.

That would be the PERFECT pistol for snipe hunting...

:neener:
 
Status
Not open for further replies.

Similar threads

G
  • Locked
Basic physics question: Rifling twist rate affected by barrel length?
Replies
19
Views
1K
JohnBT
J
chicharrones
  • Locked
Rifling Twist Rate and Barrel Length
Replies
5
Views
527
chicharrones
chicharrones
6
  • Locked
Data point for twist rate
Replies
1
Views
561
MachIVshooter
MachIVshooter
X
  • Locked
twist vs velocity
Replies
16
Views
626
michael5446
M
N
  • Locked
AR Barrel twist: Bullet stability or accuracy
Replies
23
Views
678
Bartholomew Roberts
B
Back
Top